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Trivalent chromium passivation is used after zinc plating for enhancing corrosion resistance of parts. In the passivating process, the amount of dissolved metal ions (for example zinc and iron) in the passivation solution increases the longer the solution is used. This results in a reduced corrosion resistance at elevated temperatures. Adding a top coat after this process improves the corrosion resistance but has an increased cost. To combat this, we strove to clarify the mechanism of decreased corrosion resistance and to develop a trivalent chromium passivation with a higher corrosion resistance at elevated temperatures. At first, we found that in parts produced from an older solution, the passivation layer has cracks which are not seen in parts from a fresh/new solution. These cracks grow when heated at temperatures over 120 degrees Celsius.

Evaporator surface treatment technology is important for air conditioners in that it provides resistance to rust, odor and bacteria, while imparting hydrophilicity. However, since the first-layer chromate coating contains hexavalent chromium, an environmental loading substance whose stricter regulation is under consideration, development of an alternative technology is needed. We selected titanium coating for the first-layer non-chromate coating, and added an inhibitor to the second-layer multifunctional resin coating to impart a self-healing effect without undermining the other functions. As a result, we succeeded in developing a non-chromate surface technology that ensures the same level of rust resistance and other qualities as with conventional technology.